The present invention relates to a method and apparatus for self-cleaning of heat exchangers of the shell-tube type in which transfer of heat is effected between two media, one of the media being conveyed through sets of tubes connected in parallel and the other media passing through the space between the tubes. More particularly, the present invention relates to a method and apparatus for cleaning the inner walls of the tubes of such shell-tube type heat exchangers.
As is well known to persons skilled in the art, the efficiency of a heat exchanger of the shell-tube type is unavoidably lessened after some time of operation due to deposits on the tube walls, especially to deposits along the inner tube walls. Such deposits may be caused by mechanical impurities carried by the media flowing through the tubes which condense along the tube walls or by substances contained in the media in a state of solution but percipitated therefrom by thermal and/or chemical influences. These deposits impede the heat transition to transfer through the tube walls and thereby deteriorate the efficiency of the heat exchanger. When this efficiency is lowered to a certain fraction of the original efficiency thereof, the tubes have to be cleaned mechanically and/or chemically to restore the original efficiency. As can be appreciated, having to take the heat exchanger out of operation to accomplish this cleaning necessarily lessens the economic efficiency of the apparatus in which the heat exchanger is employed, and thus tends to increase the cost of operation of the unit.
It is desirable in many instances to recover and utilize for a useful purpose hot gases generated by combustion or other plant operation which might otherwise simply be exhausted to the atmosphere. For example, in foundry operations, significant amounts of heat are generated in the melting furnace or cupola. It has been found that this heat can be used effectively and for a useful purpose as for instance, in heating a second fluid, such as water which may then be utilized for space heating of the plant. Normally, the hot gases generated in the furnace are directed through air pollution and filtering systems for removing entrained ash, molten slag or other condensable fumes which necessarily results in a cooling of the gases such that it is often not possible to utilize efficiently such gases in a heat exchanger. Accordingly, it has been found desirable to pass the hot dirty gases through a heat exchanger prior to conduction through air pollution and environmental filtering systems. However, passing of such dirty gases through tubes in a shell-tube type heat exchanger has generally been found to result in significant condensation of the condensable fumes and accumulation to the entrained ash and molten slag on the inner surfaces of the tubes which, as noted above, reduces the heat transfer efficiency of the heat exchanger.
Because it is desirable to pass the hot dirty gases through the tubes prior to air pollution environmental filtering systems, the amount of accumulation of deposits on the inner tube walls is necessarily amplified and greatly increased over accumulations found in other applications which use relatively clean gases. Thus, to provide for efficient operation of the plant and in particular of the heat exchanger in which hot dirty gases pass, it is found preferable to provide some means for self-cleaning of such heat exchangers, either continuously during operation, or intermittently, without necessitating a shut down of the heat exchanger. In the past, to clean such heavy deposits on the inner tube walls, it has been suggested that particulate cleaning media or matter be introduced into the inlet chamber for the tubes and to then flow through the tubes to scour the inner walls to remove the accumulation of slag and/or condensed metal fumes.
However, introduction of such particulate cleaning matter in the inlet chamber causes such particles to become entrained in the hot dirty gases which flow at high speeds through the tubes. This results in the major portion of the entrained particles flowing through the central portion of the tubes because of the flow velocity distribution of the gases through the tubes. Further, to the extent that any of the particulate cleaning matter is directed against the inner walls of the tubes, such particles flow at too high of a velocity which thereby creates erosion of the tube surfaces with a consequent wearing out or through of the tubes. Thus, use of particulate cleaning particles in the past has not proved efficient for cleaning of the inner tube walls in heat recovery systems which utilize the flow of hot dirty gases through the tubes of a shell-tube type heat exchanger.